The present invention relates generally to a power line communication system for an electronic ballast. More particularly, the invention relates to a power line communication system that communicates a dimming level to an electronic ballast.
As is known in the art, electronic ballasts are utilized to generate and control the amount of power consumed by gas discharge lamps. The dimming level determines the power output of the electronic ballast and therefore the lighting intensity of the lamp. This dimming level may be communicated to a dimming interface associated with the electronic ballast. The dimming interface receives a signal having dimming level information and generates a ballast dimming level signal in accordance with the communicated dimming level information. The dimming interface signal then causes the electronic ballast to operate at a particular dimming level. In this manner, a user can control the power consumed by the lamp.
Often, it is advantageous to communicate the lamp dimming level over the AC power signal that powers the electronic ballast. A power line controller is utilized to create disturbances on the AC power signal to communicate the dimming level to the electronic ballast. These disturbances are often termed notches and may be generated at a predetermined phase angle on the AC power signal, such as a zero-crossing. A power line controller creates these notches on the AC power signal by opening or closing the transmission lines.
A dimming interface associated with the electronic ballast receives the AC power signal after the power line controller has created the notches on the AC power signal. The dimming interface translates these notches into a ballast dimming level signal that corresponds to the desired dimming level of the ballast. Translating these notches involves detecting the presence or absence of the notches to determine the data bit values being transmitted to the dimming interface. For example, the presence of a notch at the zero crossing of the AC power signal may represent a “one” while the absence of a notch at the zero crossing of the AC power signal may represent a “zero”. Upon receiving these data bits, the dimming interface groups these into a bit word that represents the dimming level and creates the ballast dimming level signal.
Unfortunately, prior art communication systems require expensive components, such as processors and digital-to-analog converters, to convert the word of bits into a ballast dimming level signal. Also, because the prior art systems represent the dimming level in a digital bit word, the communication system can only represent a discrete number of dimming levels. The number of dimming levels of the lamp is therefore limited by the size of the bit word. An inherent limitation of this type of communication system is thus the frequency of the AC power signal. For example, because the notches are generally placed on the zero crossings of the AC power signal, a 120 Hz AC power signal has a transmission rate of either 120 Hz per second of both the negative and positive zero crossing are used and 60 Hz if only one zero crossing is used. Consequently, the transmission time for the dimming level increases as the number of dimming levels increases.
What is needed, then, is a communication system that does not need to translate a word of bits to generate a ballast dimming level signal.